What happened
On May 28, 1981, a Piper PA-28 RT-201 Arrow, registration HB-PDN, departed from Lausanne-La Blécherette airport on a visual flight rules (VFR) trip to Sion. The aircraft was piloted by a private pilot accompanied by his son, who also held a private pilot license. Pre-flight inspections were conducted thoroughly with no anomalies detected.
Less than one minute after takeoff, while climbing over the Valency park area and reducing power for the initial climb segment, the Avco-Lycoming IO-360 engine began to misfire and then stopped completely. The pilot maintained airspeed between 75 and 80 knots with flaps extended at the takeoff setting of 25 degrees. Two attempts to restart the engine were unsuccessful.
Facing a critical altitude loss over densely built-up areas, the pilot assessed available landing sites. Lake Geneva was too distant. Three nearby football fields in Vidy were occupied by sports activities. The only viable option was the N1 highway (Chaussée Jura), which happened to be clear of traffic at that moment. The pilot aligned with the roadway and executed a forced landing. Upon touchdown, the right wing struck a large bush on the road shoulder, causing the wing spar to snap. The aircraft skidded, pivoted nearly 180 degrees, and came to rest in a ditch with the left wing resting on an embankment. Both occupants exited the wreckage uninjured.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the aircraft, the engine, and the maintenance records. The Piper Arrow HB-PDN held a valid airworthiness certificate, and weight and balance were within limits. The pilot was qualified and fit for duty. Weather conditions were favorable with good visibility.
Post-accident testing of the Avco-Lycoming IO-360 C1C6 engine revealed irregular operation at low power settings and significant vibration before stalling. During the inspection of the Bendix Servo Fuel Injector (RSA-5AD1), investigators found that the retaining nut for the spring seat (Part Number 2539449) was missing. Without this nut, the return spring and support cup were no longer secured to the diaphragm, causing the regulator to restrict fuel flow excessively.
The investigation noted that a Service Bulletin (Lycoming RS 68) had been applied to this aircraft in September 1979, just months before the accident. This bulletin required crimping the thread above the nut to secure it. However, the expert analysis suggested this method was inadequate for such a small component, as the degree of crimping is difficult to verify and control. A later Bendix Service Bulletin (RS-71) had since recommended using Loctite 640 adhesive instead, highlighting the flaw in the previous procedure.
Findings
The primary cause of the accident was an engine failure due to a malfunctioning fuel injection regulator. The regulator failed because the retaining nut for its internal spring mechanism had become detached. This detachment was likely caused by an inadequate securing method prescribed in the maintenance service bulletin, which relied on crimping rather than a more reliable locking mechanism.
Contributing factors included:
- The inadequacy of the thread-crimping procedure specified in Service Bulletin RS 68 for securing small retaining nuts.
- The loss of the retaining nut during operation, which disabled the regulator's ability to maintain proper fuel pressure.
- The lack of sufficient altitude to reach Lake Geneva or other safe landing zones over urban terrain.
Safety action
The investigation highlighted that Bendix had issued a revised Service Bulletin (RS-71) in October 1979, recommending the use of Loctite 640 adhesive to secure the nut. This change was implemented to address the safety concerns associated with the crimping method used in RS 68. The incident underscored the critical importance of verifying the security of small fasteners in vital engine components during maintenance.